The invention relates to a method and apparatus for mass analysis by multi-pole mass filters in which the ions are subjected to a mass separation by an alternating multi-pole electric field of high frequency within a multi-pole mass filter, preferably a quadrupole mass filter by PAUL, having a stability diagram a=f(q) wherein a and q are parameters dependent on the field voltages at the poles of the multi-pole field (Paul, Raether, Z. Physik 152, page 262, 1955 and Paul, Reinhard, von ZAHN, Z. Physik 152, page 143, 1958). Mass separation is based on the fact that the paths of the ions in the electric field of such mass filters are mass-dependent.
Mass filters of the character described mostly suffer from a serious loss of transmission efficiency towards the range of high masses.
In order to overcome this disadvantage BRUBAKER (U.S. Pat. No. 3,129,327) has proposed to combine with the four primary electrodes four auxiliary electrodes at the entrance of the quadrupole mass filter to produce a decrease in the ratio of the D.C. to A.C. voltage of the quadrupole electric field in the vicinity of said entrance.
The problem underlying the invention is to provide a method and apparatus as defined above by which a high resolution combined with a satisfying sensitivity for heavy masses is obtainable.
In the solution of this problem, the invention proceeds from the consideration that the ions at the exit of the quadrupole field have different energies depending on the location of the working point within the stability diagram a=f(q) of the differential equations defining the trajectories of the ions within the quadrupole field and that especially the kinetic energy of those ions the trajectories of which are corresponding to working points near the upper margin of the triangular area of stability within the stability diagram is essentially higher than the energy of ions following trajectories more away from said upper margin. Therefore, it is possible to separate ions belonging to certain working points (a,q) near the upper margin of the stability diagram by the help of an energy discriminating field succeeding to the exit of the quadrupole field. In particular it has been observed that ions the trajectories of which within the quadrupole field are located at the margin of the stability diagram have energies characteristically different from the energies of ions at working points more distant from said margin. The energies of said ions are higher than those of other ions, so that it is possible to separate these ions from the other ions, i.e., by a retarding field at the exit of the quadrupole field. Now, as the location of the working point within the stability diagram is mass-dependent such a separation method can be used to realize a mass spectrometer or narrow-band mass filter.